Repeated passage of heavier and faster train convoys produces wear and cold rolling of the bearing surface of the rails thus causing the formation of undulations of varying lengths on the said bearing surface. This defect is remedied by truing the heads of installed track rails by means of several grinding elements optionally working in association with other contact elements, such as shoes or rollers, and forming a train driven as mentioned above, the use of a single abrading element not being sufficient to correctly and rapidly true the extensive lengths of tracks actually in service.
Two types of suspension of such contact elements have so far been used. The first type is the individual suspension, one for each contact element, the other is the suspension of several rigidly associated contact elements, optionally grouped.
The first of the above-mentioned types of suspension has the inconvenience that the contact elements, particularly the grinding elements, follow the section of the rail provided the length of the undulations is sufficient to allow the elements to dip into them whereas, in the second type, rigid groups are provided of a length such that their contact elements straddle the long undulations, making it possible for the grinding elements to restore a rectilinear section by grinding the crests. This is however only correct for a rectilinear rail whose bearing surface lies in a plane.
When a straight track presents a change in slope connected by a curve, it has a convex or concave curvature, that is a dip or a hump, or else when a track curves which, because of the inclination resulting from banking, has the effect of giving its bearing surface a conical shape, it is obvious that a series of abrading elements whose points of contact are located in a common plane will not be able to correctly effect a proper truing operation.